One of the projects I've been working on is with ReactionGrid using the physics engine. We'd been having a problem where sometimes llPushObject() was being applied to an object but dampened. It wasn't every time, but often enough to be a problem.
This only seemed to happen on the first time it was tried after a break of at least a few minutes. Do it several times in a row and only the first one might be dampened. Go get a coffee. When you tried it again, the first one might be dampened.
What works is applying a small push backward, then llSleep(0.1), then applying the desired push.